The mechanisms for the release of atrial natriuretic peptide are not clear. We have previously shown that hypophysectomy inhibits the release of ANP following an acute blood volume expansion. The effects of TRH and the importance of thyroid hormones for the basal levels and release of ANP have been evaluated. The plasma levels of ANP in the basal state and following an acute blood volume expansion are reduced during hypothyroidism but the responsiveness, i.e. the magnitude of increase following the blood volume expansion, is unchanged. Hyperthyroidism does not alter basal or stimulated plasma levels of ANP. Acute TRH administration increases ANP levels in parallel with the increase in blood pressure, but does not alter the responsiveness to acute blood volume expansion. The metabolic state induced by thyroid hormone may modulate the basal levels of ANP but does not alter the release. TRH does not appear to modulate the release of ANP apart from its effect on the sympathetic nervous system.